VASCULAR CRYPTOGAMS. 
veins of the leaf, and especially on the under side of the lamina ; but in the Acro- 
stichacese they spring both from the veins and from the mesophyll ; in Olfersia they 
cover both surfaces of the leaf at the sides of the mid-rib, or in Acrostichum only the 
under side. When, as is usually the case, the veins are the only parts that bear the 
sporangia, the fertile veins may be like the sterile ones, or they may undergo a variety 
of changes at the spots where they bear the sori ; they may be swollen into a cushion 
(forming a placenta), or they may project beyond the margin of the leaf, as in the 
Hymenophyllacese. The sorus may be seated on the end of a vein, which then 
frequently puts out two branches in the angle of which is placed the sorus, or it may 
be formed upon the vein and behind its end ; or the sorus may run for a consider- 
able distance by the side of the vein. Sometimes the fertile veins run close to the 
margin of the leaf, in other cases close to the mid-rib of the lamina. 
The Development of the Sporangium'^ is accurately known only in the Poly- 
podiaceae ; it arises there from a papillose outgrowth of one of the epidermal cells 
Fig. ■>,o\.—Aspidium Filix-'.nas. A transverse section of a leaf with a sorus consisting of the sporangia s and the 
indusium i t; right and left in the mesophyll of the leaf are two small fibro-vascular bundles, the sheath of which shows 
the dark brown thickenings on the walls that face inwards. B a young sporangium, its annulus standing vertically to 
the plane of the paper, r its apical cell ; in the interior four cells are seen resulting from the division of the central cell ; 
C lateral view of a nearly ripe sporangium, rr its annulus, a? the stalked gland peculiar to this species; within the sporan- 
gium are seen the young spores already formed. 
from which the sorus originates. Reess has shown that before the formation of 
the sporangium the epidermal cell concerned has been already divided cross-wise ; 
the papilla is cut off by a septum, another septum arising, after further elongation, 
in the mother-cell of the sporangium thus formed ; the lower cell forms the pedicel, 
the upper cell the capsule of the sporangium. The pedicel is usually transformed. 
^ When the first sporangia are ripening, all stages of development of the younger ones may 
be found in the same sorus side by side. [The following are the more important works on the 
subject : Reess, Zur Entwickelungsgeschichte des Polypodiaceen-Sporangiums, Jahrb. f. wiss. Bot. 
V, and Bot. Zeit. 1867. — Tschistiakoff, Die Sporangien und Sporen der Polypodiaceen, Nuov. Giorn. 
bot. Ital. VI.— Russow, Vergl. Unters. Petersburg 1S72. — Fischer von Waldheim, Ueb. die Ent- 
wickelung der Farnsporen, Jahrb. f. wiss. Bot. IV. — Kny, Ueb. Ceratopteris thalictroides, Bot. Zeit. 
1874. — Prantl, Die Hymenophyllaceen, 1875, and Die Schizseaceen, 188 r.] 
